Breaktru Forum
eCigarette Forum => Modding => Topic started by: carlsoti on December 16, 2014, 06:32:51 PM
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Is this a good mosfet for an unregulated dual 18650 parallel box?
http://www.digikey.com/product-detail/en/IRLB3813PBF/IRLB3813PBF-ND/2118485
If so, what would be a good resistor for it?
Would I see much of a benefit from running parallel mosfets?
What would be a good resistor for that?
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http://www.irf.com/product-info/datasheets/data/irlb3813pbf.pdf
Check out page 6, figure 12. Around 4.2 volts (batteries fully charged) you would be ok. The resistance is at about 2.5 mohms. As your batteries voltage drop and reach about 3.5 volts, the mosfet will no longer turn on. The resistance really only starts to level out around 6-10 volts. So that mosfet could be used with a charge pump/gate driver/voltage doubler or batteries in series (6-8.4v)
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Thanks for your input. Since I had the parts lying around, I decided to build with it. What's the worst that could happen? A dead short leading to fire that kills my entire family and sparks a twisted chain of events that leads to the downfall of civilization as we know it? Yeah, that's about the worst that could happen. But the actual result was nothing of the sort. The box actually works pretty well. I change batteries often, so I'm not terribly worried about the unintentional LV cutoff.
If I wanted to get the "best" mosfet (Or mosfets in parallel for lower on resistance at normal vaping voltages) possible, guess I'm looking at SMT parts, which I'm cool with. But which one?
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The worst that can happen really isn't that bad, the MOSFET can overheat and burn out. The only time that could be a real hazard is if it's close enough to the battery to overheat it through contact.
You can see some benefit from running parallel MOSFETs if you're drawing a high current. For an atomizer above an Ohm it's not going to make much difference. In general, two parallel MOSFETs halves the power loss and thereby halves the component heating, but if it's not a big amount in the first place, twice of something small is still something small. Power loss for a MOSFET is the square of current times on-state resistance. If that current is not very big then power loss is also not very big.
The best ones with the lowest on-state resistance and gate-source threshold use a surface mount package. The best P-channel I've found is the Vishay Si7157DP. The best N-channel is the TI CSD17312Q5. That would be for a single cell or parallel cells. For series cells you can use something with a higher gate-source threshold voltage which will provide a lower on-state resistance. The through-hole parts can be found with low on-state resistance in higher threshold voltages. N-channel parts generally have lower on-state resistance than P-channel parts.
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Edited: Sorry posted in wrong thread
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Craig- Do you have a link to the correct resistor for running the parallel battery/fet configuration? Also, is it generally difficult to solder these surface mounted mosfet's if its not being soldered to a pcb? I'm planning an unregulated dual 18650 box and if its easy to solder i'll go with the TI n mosfet. Will I need two of these for running resistances in the .2-.5 range?
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You can use one resistor for two parallel MOSFETs, it's not something that critical. Any value between 4.7k and 47K should be fine. You can even go with a higher value if you have to.
An SMD resistor can be soldered to leads on a TO-220 package or similar. Shouldn't be a problem. Probably an 0805 size resistor would be good. Otherwise, a 1/4W leaded resistor can be used.
Go to Mouser (http://www.mouser.com) and search on those part numbers to see listings and data sheets for them.